1. Field of the Invention
The present invention concerns an X-ray generator radiating an X-ray beam in which the radiation intensity is modulated. In particular, it relates to an X-ray generator that can form a new radiation field by bundling a plurality of X-ray tubes so that the radiation field of the X-ray beam, which is formed by a plurality of individual X-ray beams from the plurality of X-ray tubes, is narrow and wherein the radiation intensity for each of the individual X-ray beams can be set individually. Even more particularly, the present invention relates to an X-ray generator which is suitable for use in an X-ray therapeutic apparatus, said X-ray therapeutic apparatus comprising one or more of the X-ray generators.
2. Description of the Related Art
Intensity modulated radiation therapy (hereinafter abbreviated to IMRT) can reduce a radiation dose to normal tissues around the lesion portion by changing such factors as a radiation angular degree, a radiation field, and a radiation intensity of radioactive rays according to a shape of the lesion portion, so that the radioactive rays concentrate on the lesion portion. The apparatus for IMRT which modulates intensity using a multi-leaf collimator placed between a radiation aperture and a patient is known (for example, as illustrated to Japanese Unexamined Patent Application Publication No. H09-239044, Japanese Unexamined Patent Application Publication No. H10-071214, Japanese Unexamined Patent Application Publication No. 2002-153567, and Japanese Unexamined Patent Application Publication No. 2001-070466).
A therapeutic apparatus as claimed in Japanese Unexamined Patent Application Publication No. H09-239044, as shown in FIG. 15, comprises a multi-leaf collimator 13 controlled by a gantry 11 and a multi-leaf collimator control unit 12. The therapeutic apparatus can obtain a dose distribution corresponding to the three-dimensional shape of the tumor of the patient by irradiating the patient with radiation beams while changing a shape of the radiation field of the multi-leaf collimator 13 in accordance with the rotation angle of the gantry 11.
A method of obtaining the dose distribution such that the intensity of the radiation beams changes optimally is described in Japanese Unexamined Patent Application Publication No. H10-071214. The method divides a three-dimensional intensity map of the radiation field into multiple sections of intensity, and makes an intensity map of the radiation beam for every section. The intensity map is sliced to a matrix showing whether the radiation is required or not. The shape of the aperture of the collimator is set according to the matrix. Afterward, an object is irradiated with X-rays. This radiation apparatus obtains the optimum dose distribution by repeating this process.
A method of obtaining the dose distribution increasing the resolution of the radiation dose at the boundaries of a treatment area is described in Japanese Unexamined Patent Application Publication No. 2002-153567. The method divides the treatment area into a plurality of cells having a predetermined treatment intensity level. For those cells including critical tissues at the boundaries of the treatment area, the leaf of the multi-leaf collimator is moved to a position at an edge margin which sets a position in the middle of the cell and not at the edge of the cell, while radioactive rays are radiated.
A method of irradiating an object with radioactive rays described in Japanese Unexamined Patent Application Publication No. 2001-070466 resolves further a two-dimensional radiation intensity distribution quantized by levels of predetermined positive integers into a plurality of two-dimensional radiation intensity distributions of relative intensity 1. In each two-dimensional radioactive ray distribution generated by such resolving, there are only shielded object cells and radiation object cells, and the radiation intensity to a radiation object cell is the same (relative intensity 1). After this, the multi-leaf collimator is placed according to the generated two-dimensional radioactive ray distribution, and the radiation rays are radiated.
However, each invention described in Japanese Unexamined Patent Application Publication No. H09-239044, Japanese Unexamined Patent Application Publication No. H10-071214, Japanese Unexamined Patent Application Publication No. 2002-153567 and Japanese Unexamined Patent Application Publication No. 2001-070466 describes a method of obtaining cumulatively the intensity distribution of the required radiation dose by repeating the irradiation while changing the shape of the aperture of the multi-leaf collimator. According to the invention of Japanese Unexamined Patent Application Publication No. H09-239044, a dose distribution of modulated radiation intensity in three-dimensional space can be obtained by rotation of the gantry. However, as the two-dimensional plane radiation intensity is the same, intensity modulation in the two-dimensional plane cannot be obtained. Therefore, the accuracy of dose distribution obtained in the three-dimensional space is not sufficient.
The invention described in Japanese Unexamined Patent Application Publication No. H10-071214, Japanese Unexamined Patent Application Publication No. 2002-153567 and Japanese Unexamined Patent Application Publication No. 2001-070466 can obtain an intensity modulation in the two-dimensional plane by multiple irradiation, but the irradiation has to be performed at least as many times as the intensity level requires. When there is a plurality of shielded areas in the direction in which the leaf moves, even if there is only one intensity level, the leaf has to be moved and the irradiation performed multiple times.
In late years, with improvements in imaging diagnostic technology such as CT scanner, a three-dimensional shape of the lesion portion can be grasped in detail. With this, it becomes required to raise an accuracy of the radiation therapy adapting for a symptom of each part of the lesion portion. However, when an accuracy of the radiation therapy is going to be raised by using the multi-leaf collimator, a radiation number of times will be increased, burdens of the patient increase to need time for a treatment. Even more particularly, if a treatment time becomes long, there is a limit in the conventional method of improving treatment accuracy by increasing a radiation number of times because it becomes difficult to fix a lesion portion.
It is necessary to irradiate with a rotational transfer of the gantry to get a dose distribution corresponding to the three-dimensional shape of the lesion portion in the invention described in Japanese Unexamined Patent Application Publication No. H09-239044, Japanese Unexamined Patent Application Publication No. H10-071214, Japanese Unexamined Patent Application Publication No. 2002-153567 and Japanese Unexamined Patent Application Publication No. 2001-070466. However, because the gantry is heavy, the backlashes and arcuations occur easily while the gantry rotates. As shown in Japanese Unexamined Patent Application Publication No. 2002-153567, when the resolution higher than ⅓ cm is required, the three-dimensional radiation of radioactive rays after a gantry rotational transferred has a problem in respect of accuracy.
Also, because the leaf of the multi-leaf collimator is rectangular, when the treatment area has unevenness, it is difficult to position the leaf along the bound of the treatment area. Even more particularly, there is a problem to injure normal tissues of the bound vicinity because a leaf malfunctions in the location with a rippling dose distribution in the bound part of the radiation area.
It is an object of the present invention to provide the X-ray generator which can obtain immediately a two-dimensional dose distribution which the intensity is finely modulated corresponding to the desired radiation dose for X-ray therapy to each part of the lesion portion by solving previously described problems. Even more particularly, it is an object of the present invention to provide the X-ray generator which is suitable for X-ray generator in the X-ray therapeutic apparatus.